# Patent

## Title
Data storage device adjusting range of microactuator digital-to-analog converter based on operating temperature

## Abstract

A data storage device is disclosed comprising a disk, a head, and a microactuator configured to actuate the head over the disk. The data storage device further comprises control circuitry comprising a digital-to-analog converter (DAC) configured to generate a control signal applied to the microactuator. The control circuitry measures an operating temperature and then adjusts a range of the DAC based on the measured operating temperature.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority from provisional U.S. Patent Application Ser. No. 61/968,885, filed on Mar. 21, 2014, the specification of which is incorporated herein by reference.


BACKGROUND
Data storage devices such as disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.
FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk by a microactuator, and control circuitry comprising a digital-to-analog converter (DAC) configured to generate a control signal applied to the microactuator.
FIG. 2B is a flow diagram according to an embodiment wherein a range of the DAC is adjusted based on a measured operating temperature.
FIGS. 3A and 3B are graphs illustrating an embodiment wherein the range of the DAC is decreased as the operating temperature increases.
FIGS. 3C and 3D are graphs illustrating an embodiment wherein the resolution of the DAC increases as the range decreases.



DETAILED DESCRIPTION
FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16, a head 18, and a microactuator 20 configured to actuate the head 18 over the disk 16. The disk drive further comprises control circuitry 22 comprising a digital-to-analog converter (DAC) 24 configured to generate a control signal 26 applied to the microactuator 20. The control circuitry 22 is configured to execute the flow diagram of FIG. 2B, wherein an operating temperature is measured (block 28) and then a range of the DAC 24 is adjusted based on the measured operating temperature (block 30).
In the embodiment of FIG. 2A, the control circuitry 22 processes a read signal 32 emanating from the head 18 to demodulate servo sectors 340-34N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. A servo control system in the control circuitry 22 filters the PES using a suitable compensation filter to generate a control signal 36 applied to a voice coil motor (VCM) 38 which rotates an actuator arm 40 about a pivot in order to actuate the head 18 radially over the disk 16 in coarse movements in a direction that reduces the PES. The control circuitry 22 also processes the PES to generate an input signal to the DAC 24 to generate the corresponding control signal 26 applied to the microactuator 20 in order to actuate the head 18 in fine movements. The servo sectors 340-34N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern (FIG. 1).
Any suitable microactuator 20 may be employed, such as a microactuator comprising one or more piezoelectric (PZT) elements which deflect when excited by a suitable control signal (e.g., a suitable voltage). However, certain microactuators may degenerate or even fail altogether if actuated with an excessive control signal 26; for example, an excessive control signal 26 may cause a PZT element to at least partially depolarize. In addition, the gain of the microactuator 20 may be affected by the operating temperature; for example, the gain of a PZT element may increase as the operating temperature increases, thereby increasing the sensitivity of the PZT element to an excessive control signal 26. Accordingly, in one embodiment the maximum amplitude of the control signal 26 is adjusted relative to the operating temperature so as to prevent the control signal 26 from damaging the microactuator 20.
FIG. 3A shows an embodiment where the maximum amplitude of the control signal 26 applied to the microactuator is controlled by adjusting a range of the microactuator DAC 24 relative to the operating temperature. In this embodiment, as the operating temperature increases, the range of the microactuator DAC is decreased meaning that the maximum DAC input signal will generate a lower maximum DAC output signal (e.g., a lower maximum output voltage). This is further illustrated in FIG. 3B wherein at a low operating temperature TEMP_1, the maximum DAC input signal MAX INPUT generates a corresponding high DAC output signal MAX OUTPUT_1. When the operating temperature increases for example to TEMP_2, the range of the DAC is decreased such that the same maximum DAC input signal MAX INPUT generates a corresponding lower DAC output signal MAX OUTPUT_2. Consider for example an embodiment wherein the maximum DAC input signal is an arbitrary value of 10. At the operating temperature TEMP_1, the corresponding maximum DAC output signal may be 20 volts, whereas at the operating temperature TEMP_2 the corresponding maximum DAC output signal may be 15 volts.
The embodiment of FIG. 3A shows a linear relationship (staircased) between the range of the DAC and the operating temperature; however, the range of the DAC may be adjusted using any suitable function of the operating temperature, such as a suitable polynomial function. In one embodiment the control circuitry 22 may execute a calibration procedure to measure how a gain of the microactuator changes relative to the operating temperature, for example, by measuring a stroke of the microactuator at a nominal value for the control signal 26 over different operating temperatures. The relationship between the range of the microactuator DAC and the operating temperature may then be generated based on the result of this calibration procedure.
In one embodiment, decreasing the range of the microactuator DAC 24 results in a corresponding increase in the resolution. This embodiment is illustrated in FIGS. 3C and 3D where the DAC 24 may be driven using one of eight possible DAC input values (0-7). In FIG. 3C at operating temperature TEMP_1 the MAX DAC INPUT value of 7 generates a corresponding MAX DAC OUTPUT_1, and in FIG. 3D at operating temperature TEMP_3 the MAX DAC INPUT value of 7 generates a corresponding lower MAX DAC OUTPUT_3. As illustrated in FIG. 3D, decreasing the range of the microactuator DAC 24 increases the output resolution of the DAC relative to the available DAC input settings. In one embodiment, increasing the resolution of the microactuator DAC 24 improves the performance of the servo system that actuates the head 18 over the disk 16 by decreasing the quantization error of the servo system. In one embodiment, the microactuator DAC 24 is designed with sufficient range and resolution to accommodate a corresponding typical range of operating temperatures while still ensuring adequate servo performance. In another embodiment, the control circuitry 22 may transition into a different operating mode once the operating temperature exceeds a threshold (e.g., falls below a threshold) such as executing a write verify after each write operation, or disabling write operations altogether until the operating temperature reverts to an acceptable level.
The control circuitry 22 may measure the operating temperature at block 28 of FIG. 2B using any suitable technique. In one embodiment, the data storage device may comprise a suitable temperature sensor (transducer) that the control circuitry 22 periodically samples. In another embodiment, the operating temperature may be measured indirectly, such as by evaluating an operating characteristic of the data storage device that correlates with the operating temperature. For example, the fly height of the head 18 over the disk 16 may vary based on the operating temperature, and therefore the operating temperature may be measured by monitoring the fly height and/or a change in fly height and/or a change in a fly height control signal used to maintain a target fly height. In another embodiment, a characteristic of the VCM 38 may correlate with the operating temperature, such the resistance of the voice coil. In yet another embodiment, a characteristic of the microactuator 20 itself may be monitored to measure the operating temperature. For example, the stroke of the microactuator 20 may vary relative to the operating temperature and therefore in one embodiment the control circuitry 22 may periodically measure the stroke of the microactuator (e.g., by reading a test pattern on the disk while applying a periodic control signal 26 to the microactuator) to thereby measure the operating temperature.
Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.
In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.
While the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.
The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.
While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.

## Claims

1. A data storage device comprising:
a disk;
a head;
a microactuator configured to actuate the head over the disk; and
control circuitry comprising a digital-to-analog converter (DAC) configured to generate a control signal applied to the microactuator, the control circuitry configured to:
measure an operating temperature; and
adjust a range of the DAC based on the measured operating temperature.

2. The data storage device as recited in claim 1, wherein the control circuitry is further configured to decrease the range of the DAC when the measured operating temperature increases.

3. The data storage device as recited in claim 1, wherein adjusting the range of the DAC adjusts a resolution of the DAC.

4. The data storage device as recited in claim 3, wherein the control circuitry is further configured to increase the resolution of the DAC when the measured operating temperature increases.

5. The data storage device as recited in claim 1, wherein a gain of the microactuator increases when the operating temperature increases.

6. The data storage device as recited in claim 5, wherein the control circuitry is further configured to limit a maximum amplitude of the control signal to prevent the control signal from damaging the microactuator.

7. A method of operating a data storage device comprising, the method comprising:
generating a control signal using a digital-to-analog converter (DAC);
applying the control signal to a microactuator configured to actuate a head over a disk;
measuring an operating temperature; and
adjusting a range of the DAC based on the measured operating temperature.

8. The method as recited in claim 7, further comprising decreasing the range of the DAC when the measured operating temperature increases.

9. The method as recited in claim 7, wherein adjusting the range of the DAC adjusts a resolution of the DAC.

10. The method as recited in claim 9, further comprising increasing the resolution of the DAC when the measured operating temperature increases.

11. The method as recited in claim 7, wherein a gain of the microactuator increases when the operating temperature increases.

12. The method as recited in claim 11, further comprising limiting a maximum amplitude of the control signal to prevent the control signal from damaging the microactuator.